There is a hydraulic shovel as an example of a typical construction machine. Generally, a hydraulic shovel has a boom, an arm attached at an extreme end of the boom, and a bucket attached at an extreme end of the arm. The boom, the arm and the bucket are driven by hydraulic cylinders. The boom is driven by a boom cylinder provided to the boom, the arm is driven by an arm cylinder provided to the arm, and the bucket is driven by a bucket cylinder provided to the bucket.
During a work by the hydraulic shovel, a hydraulic pressure is supplied to these hydraulic cylinders and the bucket is lifted by the boom and the arm. The bucket, the arm and the boom are heavy loads, and when these are raised, a considerable potential energy is generated. Accordingly, if this potential energy is recoverable, the energy efficiency of a work by the hydraulic shovel can be improved.
Thus, there is suggested a method of recovering potential energy of an attachment by providing an assist cylinder to a boom and connecting the assist cylinder to an accumulator (for example, refer to Patent Document 1).
Moreover, there is suggested a method of recovering potential energy of an attachment by providing assist cylinders to a boom and an arm and connecting the assist cylinders to an accumulator (for example, refer to Patent Document 2).